Objective To investigate growth factors that stimulate embryonic stem cells differentiation into neurons. Abstract Recently, Thomson and his colleagues have established a stem-cell cell line from a rhesus monkey embryo at an early developmental stage. Since it has been shown that exposure to growth factors results in neuronal differentiation, we examined which growth factors are most effective in inducing the neuronal differentiation in monkey stem cells. Among those tested (basic fibroblast growth factor, epidermal growth factor, transforming growth factor a and retinoic acid), only retinoic acid at concentrations of 50 nM to 1 M consistently induced neuronal differentiation which was judged by morphology and immunocytochemcal staining with neuron specific enolase. The most effective dose of retinoic acid was 500 nM, while retinoic acid at 50 M was rather toxic to the cells. The neuronal cell type after transformation remains to be determined. Keywords retinoic acid, neuronal differentiation, embryonic stem cells